1. Field of the Invention
This invention relates to a target track position retrieval device for seeking and retrieving a target track on a rotary disc, and is applied to, for example, a disc recording/playback apparatus in which a rotary disc is rotated at constant linear velocity (CLV) to carry out recording or playback of signals.
2. Prior Art
Hitherto, in disc recording/playback apparatus adapted to rotate a disc at a constant linear velocity to carry out recording or playback of signals, for example, as in the display playback apparatus disclosed in the Japanese Patent Publication No. 50035/89, or in the track position retrieval device disclosed in the Japanese Patent Publication No. 50030/89, the number of tracks from a present track up to a target track is calculated as a function of the playback time normally needed to advance from the present track up to the target track, thus to position the head at and recover information from the target track.
Namely, in the disc playback apparatus disclosed in the Japanese Patent Publication No. 50035/89, the target track of radius [r.sub.2 ] may be calculated, if it is assumed that the disc is provided with concentrical tracks having a track pitch [p], that the playback time needed to advance from a track of a radius [r.sub.1 ] to the track of radius [r.sub.2 ] is [t] and that the linear velocity of the disc is represented by [v], as shown in FIG. 1 herein, in accordance with the following equation (1): ##EQU3## Further, the number of tracks [n] from the track of radius [r.sub.1 ] up to the track of radius [r.sub.2 ] is determined by equation (2): ##EQU4## Retrieval of the target track is carried out as a function of n.
In the target track retrieval device disclosed in the Japanese Patent Publication No. 50030/89, signal playback means plays back signals from information tracks recorded on a rotary disc rotationally driven at constant linear velocity, movement means moves the playback means in a direction traversing information tracks on the rotary disc, counter means responds to the playback means output to count the number of tracks traversed by the playback means, and control means compares predetermined target position information which indicates a target track position to the count value of the counter means to stop movement of the playback means when the predetermined target position information and the count value are in correspondence with each other. If the disc is as shown in FIG. 2 herein, let it be assumed that an innermost circumferential track radius is represented as [r.sub.o ], the angular velocity of the disc when the innermost circumferential track is played back is represented by [N.sub.o ], the track pitch is represented by [p], the radius of an arbitrary track is represented as [r] and the time duration for playback from radius r.sub.0 to radius r is represented by [t]. These parameters are related in accordance with equation (3): ##EQU5## Further, the number of tracks [M.sub.2-1 ] from a present track position of radius r.sub.1 to a target track position of radius r.sub.2 is determined by the following equation (4): ##EQU6## Retrieval of the target track is carried out as a function of the number of tracks M.sub.2-1. Further, at the target track, the angular velocity [N] of the disc rotated for constant linear velocity is determined by the following equation (5): ##EQU7## By controlling disc rotational velocity before the target track is reached, high speed retrieval of the target track can be obtained.
Meanwhile, in conventional target track retrieval apparatuses as described above, subduplicate processing and multiplicative processing are required for calculating the number of tracks from a present track position up to a target track position, and for calculating disc velocity at the target track. Accordingly, when a low cost processor is used to implement high speed retrieval of the target track, a large amount of computation time is needed and this is an undesired drawback.
Further, it might be thought that a ROM can be used to store a data table in which the number of tracks up to selected target tracks and the velocity of the disc at those target tracks are calculated in advance. However, in accordance with the standard requirements of, e.g., the so-called compact disc, the linear velocity [v.sub.o ] of the disc admits of a tolerance in the range 1.2 to 1.4 [m/s], and the track pitch [p] may be in the range 1.5.times.10.sup.-6 to 1.7.times.10.sup.-6 [m]. Because of these ranges, the necessary data table that is required must account for a plurality of linear velocities [v.sub.o ] and track pitches [p] and thus must be very large. Consequently, it is difficult to use a data table that will provide high target track acquisition accuracy because of practical limits on the ROM capacity.